Liquid crystal display apparatuses have widely been used for display units, projectors, mobile phones, handheld information terminals, and the like.
Such a liquid crystal display apparatus is a non-self-luminous display apparatus that reproduces an image by utilizing light applied from either (i) a light source provided inside the apparatus or (ii) the outside of the apparatus.
In the non-self-luminous display apparatus, the aforesaid light is applied to the display panel having pixels that are arranged in a matrix manner with predetermined intervals. A drive signal (drive voltage) corresponding to a displayed image (including texts) is separately supplied to each pixel.
The supply of the drive signal varies the transmittance (or reflectance) at each pixel part of the display panel. This makes it possible to partially modulate the intensity of display light, and allows the panel to display an image thereon.
There are two types of such a non-self-luminous display apparatus: a direct-view display apparatus that allows the user to directly view an image reproduced on the display panel; and a projection display apparatus (projector) that magnifies an image on the display panel and projects the image on a screen, using a projection lens.
Apart from the liquid crystal display apparatus, known examples of the non-self-luminous display apparatus include an electrochromic display panel, an electrophoretic display panel, toner display panel, and a PLZT panel.
Furthermore, there are three types of the aforesaid display apparatuses: a reflection type; a transmission type; and a semi-transmission type.
A reflection-type display apparatus allows light (outside light) to enter the inside of the display panel and reflects the light on a reflection layer, so as to obtain display light.
A transmission-type display apparatus is arranged in such a manner that light from a light source (backlight) provided behind the display panel is emitted to the outside via the display panel.
In a poorly-illuminated place, a semi-transmission-type display apparatus reproduces an image in a transmissive manner, using light emitted from the backlight. On the other hand, in a well-illuminated place, the semi-transmission-type display apparatus reproduces an image using ambient light. In this way, an image with a high contrast ratio is reproduced irrespective of the brightness of ambient light.
In a liquid crystal display apparatus, the optical characteristics (transmittance and reflectance) of a liquid crystal layer in a liquid crystal display panel (liquid crystal panel) are varied by applying the above-described drive signal to pixels.
In this relation, there are two ways of separately applying a drive signal to each pixel: simple matrix and active matrix.
In the active matrix, switching elements and wiring for supplying a drive voltage to pixel electrodes must be provided on the liquid crystal panel. Examples of the switching element are a non-linear two-terminal element such as an MIM (Metal-Insulator-Metal) element and a three-terminal element such as a TFT (Thin Film Transistor) element.
The switching element (especially the TFT element) thus described has such a characteristic that the resistance of the element in the off state decreases upon receiving intense light. On this account, when voltage is applied, an electric charge charged in a pixel capacity (picture element capacity) is discharged, so that a particular state of display cannot be achieved (e.g. leakage of light occurs even in a case of black display, thereby decreasing the contrast ratio).
To prevent light from entering the TFT (channel area in particular), there is such a conventional solution that a light shielding layer (termed black matrix) is provided on a part of a TFT substrate on which TFTs and pixels electrodes are formed or on a part of an opposing substrate facing the TFT substrate with a liquid crystal layer being interposed therebetween.
This type of the light shielding layer is desirable for a reflective liquid crystal display apparatus, because a reflective electrode can be used as a light shielding layer, so that the provision of the reflective electrode does not reduce the area of effective pixels.
On the contrary, in a transmissive liquid crystal display apparatus that reproduces an image by utilizing transmitted light, no existing members can be used as the light shielding layer.
For this reason, in addition to non-transmissive TFTs, gate bus lines, and source bus lines, a light shielding layer is further provided in the transmissive liquid crystal display apparatus. This reduces the area of the effective pixels, reduces the ratio (open area ratio) between the total area of the display section and the effective pixel area, and consequently decreases the brightness.
Furthermore, the open area ratio significantly decreases as the resolution of the liquid crystal panel increases and the size of the liquid crystal panel decreases. This is because, even if the pixel pitch is shortened, the TFTs and bus lines cannot be reduced in size beyond certain levels, on account of the constraints in electrical characteristics, manufacturing technology, and the like.
In particular, semi-transmission-type liquid crystal display apparatuses, which are adopted as a small display apparatus for a handheld device such as a mobile phone, are arranged such that each pixel has an area (reflective area) where a display is created in a reflection mode and an area (transmission area) where a display is created in a transmission mode. For this reason, if the pixel pitch is shortened, the ratio between the total of the display area and the transmission area (i.e. the open area ratio of the transmission area) significantly reduces, so that the decrease of the brightness gets worse.
In a case of a direct-view liquid crystal display apparatus and a single-plate projector that utilize the absorption of light by a color filter in order to reproduce a color image, the efficiency of the use of light (i.e. brightness) further decreases.
The following is a list of prior art documents.
Document 1; Japanese Patent No. 3293589 (registered on Jun. 17, 2002)
Document 2 Japanese Laid-Open Patent Application No. 2002-62818 (published on Feb. 28, 2002)
Document 3; Japanese Laid-Open Patent Application No. 2002-42528 (published on Feb. 8, 2002)
Document 4; Journal for Information Display, Vol. 11, No. 4 2003, pp 641-645 (published in November, 2003)